Grease composition

ABSTRACT

A grease composition containing a lubricant base oil, diurea compounds represented by the following general formulas (1) to (3), and an organic molybdenum compound; wherein respective contents of the diurea compounds represented by the following general formulas (1) to (3) satisfy conditions defined by the following expressions (4) and (5); and wherein the organic molybdenum compound is contained by 0.1 to 20 mass % based on the total amount of the grease composition:  
                 
 
where R 1  is a hydrocarbon group containing an aromatic ring, R 2  is a divalent hydrocarbon group, and R 3  is a hydrocarbon group containing an aliphatic ring; 
 
5≦ W   1   +W   2   +W   3 ≦30   (4) 
 
0.1≦( W   1 +0.5× W   2 )/( W   1   +W   2   +W   3 )≦1.0   (5) 
 
where W 1 , W 2 , and W 3  are respective contents (mass %) of the diurea compounds represented by general formulas (1) to (3) based on the total amount of the grease composition.

TECHNICAL FIELD

The present invention relates to a grease composition; and, morespecifically, to a grease composition favorably used in a constantvelocity joint, a bearing for a continuously variable transmission, abearing for both a car and a railway vehicle, etc.

BACKGROUND ART

Various mechanical elements where metals contact with each other, suchas constant velocity joints for a shaft transmitting a driving forcefrom a transmission of a car to its tires, bearings for continuouslyvariable transmissions of cars and the like, and axle bearings for carsand railway vehicles are filled with grease acting as a lubricant.

The grease used in these various mechanical elements is required tosuppress the temperature rise in its early stage of use from theviewpoint of extending life in particular, and reduce the friction fromthe viewpoints of extending life and saving energy.

As a method of suppressing the temperature rise, the kinetic viscosityof a base oil of grease has been lowered in general. As a method ofreducing the friction, the use of various additives such as organicmolybdenum compounds and organic zinc compounds has been known.

DISCLOSURE OF THE INVENTION

However, it has been quite difficult for the above-mentionedconventional methods to suppress the temperature rise in the early stageof use and reduce the friction at the same time. For example, when thekinetic viscosity of the base oil is lowered in order to suppress thetemperature rise in the early stage of use, the oil film thicknessdecreases so that metals are more likely to contact with each other,whereby the friction/wear tends to increase. Further, there is a fear ofthe life shortening at a high temperature.

Also, under recent circumstances where load has been increasing asvarious mechanical elements such as constant velocity joints, bearingsfor continuously variable transmissions, and axle bearings for cars andrailway vehicles have been attaining higher performances and lighterweights, properties demanded for the grease to suppress the temperaturerise in the early stage of use and lower the friction have beenincreasing.

In view of the above-mentioned problem of the prior art, it is an objectof the present invention to provide a grease composition which canrealize, at a high level, the suppression of the temperature rise in theearly stage of use and the reduction of the friction.

The inventors conducted diligent studies in order to achieve theabove-mentioned object and, as a result, have found that a greasecomposition in which specific diurea and organic molybdenum compoundsare compounded at respective predetermined compounding ratios in alubricant base oil can suppress the temperature rise in constantvelocity joints, bearings for continuously variable transmissions, axlebearings for cars and railway vehicles, and the like in their earlystage of use and lower the friction therein, thereby completing thepresent invention.

Namely, the grease composition in accordance with the present inventionis one containing a lubricant base oil, diurea compounds represented bythe following general formulas (1) to (3), and an organic molybdenumcompound; wherein respective contents of the diurea compoundsrepresented by the following general formulas (1) to (3) satisfyconditions defined by the following expressions (4) and (5); and whereinthe organic molybdenum compound is contained by 0.1 to 20 mass % basedon the total amount of the grease composition:

where R¹ is a hydrocarbon group containing an aromatic ring, R² is adivalent hydrocarbon group, and R³ is a hydrocarbon group containing analiphatic ring;5≦W ₁ +W ₂ +W ₃≦30   (4)0.1≦(W ₁+0.5×W ₂)/(W ₁ +W ₂ +W ₃)≦1.0   (5)where W₁, W₂, and W₃ are respective contents (each expressed by the unitof mass %) of the diurea compounds represented by general formulas (1)to (3) based on the total amount of the grease composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are perspective and top plan views showing a test pieceused in a friction test, respectively.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, preferred embodiments of the present invention will beexplained in detail.

Examples of the lubricant base oil used in the grease composition of thepresent invention are mineral oils and/or synthetic oils.

Examples of the mineral oils are those obtained by a method usuallycarried out in a lubricant manufacturing process in a petroleum refiningindustry, more specifically, those obtained when a lubricant fractionyielded by distilling a crude oil under normal pressure and underreduced pressure is refined by carrying out at least one of processes ofsolvent deasphalting, solvent extraction, hydrocracking, solventdewaxing, contact dewaxing, hydro-refining, washing with sulfuric acid,clay treatment, etc.

Specific examples of the synthetic oils include poly α-olefins such aspolybutene, 1-octene oligomer, and 1-decene oligomer or theirhydrogenated products; diesters such as ditridecyl glutarate,di(2-ethylhexyl) adipate, diisodecyl adipate, ditridecyl adipate, anddi(3-ethylhexyl) sebacate; polyol esters such as trimethylolpropanecaprylate, trimethylolpropane pelargonate, pentaerythritol2-ethylhexanoate, and pentaerythritol pelargonate; aromatic ester oilssuch as trioctyl trimellitate, tridecyl trimellitate, and tetraoctylpyromellitate; complex esters which are esters formed by a mixed fattyacid of a dibasic acid and a monobasic acid, and a polyhydric alcohol;alkyl naphthalene; alkyl benzene; polyoxyalkylene glycol; polyphenylether; dialkyldiphenyl ether; silicone oil; and their mixtures.

The kinetic viscosity of the lubricant base oil at 100° C. is preferably2 to 40 mm²/s, more preferably 3 to 20 mm²/s. The viscosity index of thebase oil is preferably at least 90, more preferably at least 100.

In the present invention, diurea compounds represented by the followinggeneral formulas (1) to (3) are added as a thickener to the lubricantbase oil:

In the above-mentioned formulas (1) to (3), R¹ is a hydrocarbon groupcontaining an aromatic ring. Examples of such a group include phenylgroup, naphthyl group, alkylaryl groups in which at least one alkylgroup is added to these groups as a substituent, and arylalkyl groups inwhich aryl groups such as phenyl and naphthyl groups are added to alkylgroups as substituents.

Though the number of carbons in the hydrocarbon group containing anaromatic ring represented by R¹ is not limited in particular, one with acarbon number of 7 to 12 is preferably used. Specific examples of thehydrocarbon group containing an aromatic ring with such a carbon numberinclude toluyl group, xylyl group, β-phenacyl group, t-butylphenylgroup, dodecylphenyl group, benzyl group, and methylbenzyl group.

R² in general formulas (1) to (3) is a divalent hydrocarbon group(preferably with a carbon number of 6 to 20, a carbon number of 6 to 15in particular) Examples of such a hydrocarbon group include linear orbranched alkylene groups, linear or branched alkenylene groups,cycloalkylene groups, and aromatic groups. Among them, ethylene group,2,2-dimethyl-4-methylhexylene group, and groups represented by thefollowing formulas (6) to (14) are preferred, the groups represented bythe formulas (7) and (9) in particular:

In general formulas (2) and (3), R³ is a hydrocarbon group containing analiphatic ring. Though the number of carbons contained in thehydrocarbon group containing an aliphatic ring represented by R³ is notrestricted in particular, one with a carbon number of 7 to 12 ispreferably used.

As the hydrocarbon group containing an aliphatic ring represented by R³,cyclohexyl group or alkylcyclohexyl group is preferably used. Specificexamples include methylcyclohexyl group, dimethylcyclohexyl group,ethylcyclohexyl group, diethylcyclohexyl group, propylcyclohexyl group,isopropylcyclohexyl group, 1-methyl-3-propylcyclohexyl group,butylcyclohexyl group, pentylcyclohexyl group, pentylmethylcyclohexylgroup, and hexylcyclohexyl group, among which cyclohexyl group,methylcyclohexyl group, dimethylcyclohexyl group, and ethylcyclohexylgroup are more preferable.

The respective contents of the diurea compounds represented by generalformulas (1) to (3) are required to satisfy the conditions representedby the following expressions (4) and (5):5≦W ₁ +W ₂ +W ₃≦30   (4)0.1≦(W ₁+0.5×W ₂)/(W ₁ +W ₂ +W ₃)≦1.0   (5)where W₁, W₂, and W₃ are respective contents (each expressed by the unitof mass %) of the diurea compounds represented by general formulas (1)to (3) based on the total amount of the grease composition.

As expression (4) shows, the sum W₁+W₂+W₃ of contents of the diureacompounds represented by general formulas (1) to (3) is 5 to 30 mass %based on the total amount of the grease composition. When the sumW₁+W₂+W₃ is less than 5 mass %, the effect of the thickener is so weakthat the composition fails to become sufficiently greasy. For the samereason, W₁+W₂+W₃ is preferably at least 10 mass %. When W₁+W₂+W₃ exceeds30 mass %, the composition becomes too hard as a grease, thereby failingto exhibit a sufficient lubricating property. For the same reason,W₁+W₂+W₃ is preferably 20 mass % or less.

When (W₁+0.5×W₂)/(W₁+W₂+W₃) in expression (5) is less than 0.1, theeffect of suppressing the temperature rise in the early stage of usebecomes weaker. For the same reason, (W₁+0.5×W₂)/(W₁+W₂+W₃) ispreferably at least 0.2, more preferably at least 0.3, furtherpreferably at least 0.4. Similarly, from the viewpoint of the effect ofsuppressing the temperature rise in the early stage of use,(W₁+0.5×W₂)/(W₁+W₂+W₃) is preferably not greater than 0.7, morepreferably less than 0.55, further preferably less than 0.5.

Though urea type thickeners apt to become harder as time passes ingeneral, the grease composition of the present invention is relativelyless likely to harden. For making it further less likely to harden,(W₁+0.5×W₂)/(W₁+W₂+W₃) is preferably at least 0.3, more preferably atleast 0.35, further preferably at least 0.4, at least 0.45 inparticular.

For example, these diurea compounds are obtained when a diisocyanaterepresented by the general formula of OCN—R²—NCO and an aminerepresented by the general formula of R¹—NH₂ and/or R³—NH₂ are caused toreact against each other at a temperature of 10° to 200° C. in the baseoil. Here, R¹, R², and R³ correspond to those of (1) to (3),respectively.

When the value of (W₁+0.5×W₂)/(W₁+W₂+W₃) in expression (5) is not 1, thediurea compounds may be a mixture of a reaction product of diisocyanateand the amine represented by R¹—NH₂, and a reaction product ofdiisocyanate and the amine represented by R³—NH₂; or a reaction productof diisocyanate and a mixture of the amine represented by R¹—NH₂ and theamine represented by R³—NH₂.

The grease composition of the present invention further contains anorganic molybdenum compound in addition to the above-mentioned lubricantbase oil and diurea compounds. Examples of the organic molybdenumcompound include a phosphate or thiophosphate ester derivativerepresented by the following general formula (15) and a dithiocarbamateester derivative represented by the following general formula (16):

In general formulas (15) and (16), R may be identical or different, eachrepresenting a hydrocarbon group with a carbon number of at least 1; cpieces of X may be identical or different, each representing an oxygenatom or sulfur atom; and each of a, b, and c represents an integer of 1to 6.

Examples of the hydrocarbon group represented by R in theabove-mentioned formulas (15) and (16) include alkyl groups with acarbon number of 1 to 24, cycloalkyl groups with a carbon number of 5 to7, alkylcycloalkyl groups with a carbon number of 6 to 11, aryl groupswith a carbon number of 6 to 18, alkylaryl groups with a carbon numberof 7 to 24, and arylalkyl groups with a carbon number of 7 to 12.

Specific examples of the above-mentioned alkyl groups include methylgroup, ethyl group, propyl group (including all the branched isomersthereof), pentyl group (including all the branched isomers thereof),hexyl group (including all the branched isomers thereof), heptyl group(including all the branched isomers thereof), octyl group (including allthe branched isomers thereof), nonyl group (including all the branchedisomers thereof), decyl group (including all the branched isomersthereof), undecyl group (including all the branched isomers thereof),dodecyl group (including all the branched isomers thereof), tridecylgroup (including all the branched isomers thereof), tetradecyl group(including all the branched isomers thereof), pentadecyl group(including all the branched isomers thereof), hexadecyl group (includingall the branched isomers thereof), heptadecyl group (including all thebranched isomers thereof), octadecyl group (including all the branchedisomers thereof), nonadecyl group (including all the branched isomersthereof), icosyl group (including all the branched isomers thereof),henicosyl group (including all the branched isomers thereof), docosylgroup (including all the branched isomers thereof), tricosyl group(including all the branched isomers thereof), and tetracosyl group(including all the branched isomers thereof).

Specific examples of the above-mentioned cycloalkyl groups includecyclopentyl group, cyclohexyl group, and cycloheptyl group.

Specific examples of the above-mentioned alkylcycloalkyl groups includemethylcyclopentyl group (including all the substituted isomers thereof),ethylcyclopentyl group (including all the substituted isomers thereof),dimethylcyclopentyl group (including all the substituted isomersthereof), propylcyclopentyl group (including all the branched isomersand substituted isomers thereof), methylethylcyclopentyl group(including all the substituted isomers thereof), trimethylcyclopentylgroup (including all the substituted isomers thereof), butylcyclopentylgroup (including all the branched isomers and substituted isomersthereof), methylpropylcyclopentyl group (including all the branchedisomers and substituted isomers thereof), diethylcyclopentyl group(including all the substituted isomers thereof), dimethylcyclopentylgroup (including all the substituted isomers thereof), methylcyclohexylgroup (including all the substituted isomers thereof), ethylcyclohexylgroup (including all the substituted isomers thereof),dimethylcyclohexyl group (including all the substituted isomersthereof), propylcyclohexyl group (including all the branched isomers andsubstituted isomers thereof), methylethylcyclohexyl group (including allthe substituted isomers thereof), trimethylcyclohexyl group (includingall the substituted isomers thereof), butylcyclohexyl group (includingall the branched isomers and substituted isomers thereof),methylpropylcyclohexyl group (including all the branched isomers andsubstituted isomers thereof), diethylcyclohexyl group (including all thesubstituted isomers thereof), dimethylethylcyclohexyl group (includingall the substituted isomers thereof), methylcycloheptyl group (includingall the substituted isomers thereof), ethylcycloheptyl group (includingall the substituted isomers thereof), dimethylcycloheptyl group(including all the substituted isomers thereof), propylcycloheptyl group(including all the substituted isomers thereof), methylethylcycloheptylgroup (including all the substituted isomers thereof),trimethylcycloheptyl group (including all the substituted isomersthereof), butylcycloheptyl group (including all the branched isomers andsubstituted isomers thereof), methylpropylcycloheptyl group (includingall the branched isomers and substituted isomers thereof),diethylcycloheptyl group (including all the substituted isomersthereof), and dimethylethylcycloheptyl group (including all thesubstituted isomers thereof).

Specific examples of the above-mentioned aryl groups include phenylgroup and naphthyl group.

Specific examples of the above-mentioned alkylaryl groups include tolylgroup (including all the substituted isomers thereof), xylyl group(including all the substituted isomers thereof), ethylphenyl group(including all the substituted isomers thereof), propylphenyl group(including all the branched isomers and substituted isomers thereof),methylethylphenyl group (including all the substituted isomers thereof),trimethylphenyl group (including all the substituted isomers thereof),butylphenyl group (including all the branched isomers and substitutedisomers thereof), methylpropylphenyl group (including all the branchedisomers and substituted isomers thereof), diethylphenyl group (includingall the substituted isomers thereof), dimethylethylphenyl group(including all the substituted isomers thereof), pentylphenyl group(including all the branched isomers and substituted isomers thereof),hexylphenyl group (including all the branched isomers and substitutedisomers thereof), heptylphenyl group (including all the branched isomersand substituted isomers thereof), octylphenyl group (including all thebranched isomers and substituted isomers thereof), nonylphenyl group(including all the branched isomers and substituted isomers thereof),decylphenyl group (including all the branched isomers and substitutedisomers thereof), undecylphenyl group (including all the branchedisomers and substituted isomers thereof), dodecylphenyl group (includingall the branched isomers and substituted isomers thereof),tridecylphenyl group (including all the branched isomers and substitutedisomers thereof), tetradecylphenyl group (including all the branchedisomers and substituted isomers thereof), pentadecylphenyl group(including all the branched isomers and substituted isomers thereof),hexadecylphenyl group (including all the branched isomers andsubstituted isomers thereof), heptadecylphenyl group (including all thebranched isomers and substituted isomers thereof), and octadecylphenylgroup (including all the branched isomers and substituted isomersthereof).

Examples of the arylalkyl groups include benzyl group, phenethyl group,phenylpropyl group (including all the branched isomers thereof), andphenylbutyl group (including all the branched isomers and substitutedisomers thereof).

Specific examples of the compounds represented by the above-mentionedgeneral formulas (15) and (16) include molybdenum phosphate, molybdenumthiophosphate, molybdenum dithiophosphate, and molybdenumdithiocarbamate.

The phosphate or thiophosphate ester derivative represented by theabove-mentioned formula (15) and the dithiocarbamate ester derivativerepresented by the above-mentioned formula (16) are compounds usuallyobtained by causing a phosphate ester, a thiophosphate ester, or adithiocarbamate ester to react with an inorganic molybdenum compound(molybdenum trioxide, molybdenum oxide, salts thereof, etc.) togetherwith a sulfur source if necessary.

Since molybdenum can take various valences, the compounds obtained bythe above-mentioned reaction are usually mixtures. Among them, the mosttypical compounds are those represented by the following compounds (17)and (18):

As the organic molybdenum compound in the present invention, therespective compounds represented by the above-mentioned general formulas(15) and (16) may be used separately or in a mixture. For use in bearinggrease, the compound represented by the above-mentioned general formula(16) is more preferable because of its better thermal stability.

The content of the organic molybdenum compound in the present inventionis at least 0.1 mass %, preferably at least 0.5 mass %, based on thetotal amount of the grease composition. On the other hand, the contentis not greater than 20 mass %, preferably not greater than 10 mass %.The friction reducing effect of the grease is insufficient when thecontent of the organic molybdenum compound is less than 0.1 mass %,whereas the friction reducing effect cannot be obtained in proportion tothe amount of addition when the content exceeds 20 mass %.

For improving the resistance to fretting, it will be preferred if atleast one species of compound selected from the group consisting ofparaffin oxides and phosphorus compounds is added to the greasecomposition of the present invention. When these compounds are added, ahigher level of resistance to fretting can be achieved over a longperiod in a constant velocity joint in which micromotion wear (fretting)is likely to become problematic upon minute reciprocation in particular.

Examples of the paraffin oxides used in the present invention includeparaffin oxide, salts of paraffin oxide, and esters of paraffin oxide.Examples of paraffin oxide mentioned here include those obtained byoxidizing petroleum waxes such as paraffin wax, microcrystalline wax,and slack wax, or a synthetic wax such as polyolefin wax. Examples ofthe salts of paraffin oxide include alkali metal salts, alkaline earthmetal salts, and amine salts of paraffin oxide. Examples of the estersof paraffin oxide include esters formed between an alcohol (mostpreferably methanol) having a carbon number of 1 to 24 (preferably 1 to12, more preferably 1 to 6) and paraffin oxide. The paraffin oxides usedin the present invention may have any properties. However, from theviewpoint of the resistance to fretting, their melting point ispreferably at least 25° C., more preferably 30° C., but preferably nothigher than 110° C., more preferably not higher than 70° C. The totalacid number is preferably at least 0.2 mgKOH/g, more preferably at least1 mgKOH/g, but preferably not greater than 65 mgKOH/g, more preferablynot greater than 40 mgKOH/g.

Specific examples of the phosphorus compounds include phosphate esters,acid phosphate esters, amine salts of acid phosphate esters, chlorinatedphosphate esters, phosphite esters, and thiophosphate esters. Thesephosphorus compounds are esters formed between phosphoric acid,phosphorous acid, or thiophosphoric acid and an alkanol or polyetheralcohol, or their derivatives.

More specific examples of phosphate esters include tributyl phosphate,tripentyl phosphate, trihexyl phosphate, triheptyl phosphate, trioctylphosphate, trinonyl phosphate, tridecyl phosphate, triundecyl phosphate,tridodecyl phosphate, tritridecyl phosphate, tritetradecyl phosphate,tripentadecyl phosphate, trihexadecyl phosphate, triheptadecylphosphate, trioctadecyl phosphate, trioleyl phosphate, triphenylphosphate, tricresyl phosphate, trixylenyl phosphate, cresyldiphenylphosphate, and xylenyldiphenyl phosphate.

Examples of the acid phosphate esters include monobutyl acid phosphate,monopentyl acid phosphate, monohexyl acid phosphate, monoheptyl acidphosphate, monooctyl acid phosphate, monononyl acid phosphate, monodecylacid phosphate, monoundecyl acid phosphate, monododecyl acid phosphate,monotridecyl acid phosphate, monotetradecyl acid phosphate,monopentadecyl acid phosphate, monohexadecyl acid phosphate,monoheptadecyl acid phosphate, monooctadecyl acid phosphate, monooleylacid phosphate, dibutyl acid phosphate, dipentyl acid phosphate, dihexylacid phosphate, diheptyl acid phosphate, dioctyl acid phosphate, dinonylacid phosphate, didecyl acid phosphate, diundecyl acid phosphate,didodecyl acid phosphate, ditridecyl acid phosphate, ditetradecyl acidphosphate, dipentadecyl acid phosphate, dihexadecyl acid phosphate,diheptadecyl acid phosphate, dioctadecyl acid phosphate, and dioleylacid phosphate.

Examples of the amine salts of acid phosphate esters include saltsformed between the acid phosphate esters and amines such as methylamine,ethylamine, propylamine, butylamine, pentylamine, hexylamine,heptylamine, octylamine, dimethylamine, diethylamine, dipropylamine,dibutylamine, dipentylamine, dihexylamine, diheptylamine, dioctylamine,trimethylamine, triethylamine, tripropylamine, tributylamine,tripentylamine, trihexylamine, triheptylamine, and trioctylamine.

Examples of the chlorinated phosphate esters includetris(dichloropropyl)phosphate, tris(chloroethyl)phosphate,tris(chlorophenyl)phosphate, and polyoxyalkylenebis[di(chloroalkyl)]phosphate.

Examples of the phosphite esters include phosphite diesters such asdibutyl hydrogen phosphite, dipentyl hydrogen phosphite, dihexylhydrogen phosphite, diheptyl hydrogen phosphite, dioctyl hydrogenphosphite, dinonyl hydrogen phosphite, didecyl hydrogen phosphite,diundecyl hydrogen phosphite, didodecyl hydrogen phosphite, dioleylhydrogen phosphite, diphenyl hydrogen phosphite, and dicresyl hydrogenphosphite; and phosphite triesters such as tributyl phosphite, tripentylphosphite, trihexyl phosphite, triheptyl phosphite, trioctyl phosphite,trinonyl phosphite, tridecyl phosphite, triundecyl phosphite, tridodecylphosphite, trioleyl phosphite, triphenyl phosphite, and tricresylphosphite.

Examples of phosphorothionate include tributyl phosphorothionate,tripentyl phosphorothionate, trihexyl phosphorothionate, triheptylphosphorothionate, trioctyl phosphorothionate, trinonylphosphorothionate, tridecyl phosphorothionate, triundecylphosphorothionate, tridodecyl phosphorothionate, tritridecylphosphorothionate, tritetradecyl phosphorothionate, tripentadecylphosphorothionate, trihexadecyl phosphorothionate, triheptadecylphosphorothionate, trioctadecyl phosphorothionate, trioleylphosphorothionate, triphenyl phosphorothionate, tricresylphosphorothionate, trixylenyl phosphorothionate, cresyldiphenylphosphorothionate, xylenyldiphenyl phosphorothionate,tris(n-propylphenyl)phosphorothionate,tris(isopropylphenyl)phosphorothionate,tris(n-butylphenyl)phosphorothionate,tris(isobutylphenyl)phosphorothionate,tris(s-butylphenyl)phosphorothionate, andtris(t-butylphenyl)phosphorothionate.

The above-mentioned phosphorus compounds may be used one by one or in amixture of two or more.

Among them, because of better resistance to fretting, phosphite estersare preferable, phosphite diesters are more preferable, and diphenylhydrogen phosphite is further preferable.

The content of at least one compound selected from the group consistingof the paraffin oxides and phosphorus compounds is preferably at least0.5 mass %, more preferably at least 1.0 mass %, based on the totalamount of the grease composition. When the content is less than 0.5 mass%, the resistance to fretting tends to become insufficient in thegrease. On the other hand, the content is preferably not greater than 15mass %, more preferably 10 mass %. When the content exceeds 15 mass %,the resistance to fretting cannot be obtained in proportion to theamount of addition.

For further improving the resistance to fretting, (W₁+0.5×W₂)/(W₁+W₂+W₃)in expression (5) is preferably at least 0.3, more preferably at least0.35, further preferably at least 0.4, furthermore preferably at least0.45, but is preferably not greater than 0.7, more preferably notgreater than 0.6, further preferably less than 0.5.

For improving performances, the grease composition of the presentinvention can further contain solid lubricants, extreme pressure agents,antioxidants, oily agents, antirusts, viscosity index improvers, etc.when necessary as long as its properties do not deteriorate.

Specific examples of the solid lubricants include graphite, graphitefluoride, polytetrafluoroethylene, molybdenum disulfide, antimonysulfide, and alkali (earth) metal borates.

Specific examples of the extreme pressure agents include organic zinccompounds such as zinc dialkyldithiophosphate and zincdiaryldithiophosphate; and sulfur-containing compounds such asdihydrocarbyl polysulfide, sulfide esters, thiazole compounds, andthiadiazole compounds.

Specific examples of the antioxidants include phenol type compounds suchas 2,6-di-t-butylphenol and 2,6-di-t-butyl-p-cresol; amine typecompounds such as dialkyldiphenylamine, phenyl-α-naphthylamine, andp-alkylphenyl-α-naphthylamine; sulfur type compounds; and phenothiazinetype compounds.

Specific examples of the oily agents include amines such as laurylamine,myristylamine, palmitylamine, stearylamine, and oleylamine; higheralcohols such as lauryl alcohol, myristyl alcohol, palmityl alcohol,stearyl alcohol, and oleyl alcohol; higher fatty acids such as lauricacid, myristic acid, palmitic acid, stearic acid, and oleic acid; fattyacid esters such as methyl laurate, methyl myristate, methyl palmitate,methyl stearate, and methyl oleate; amides such as laurylamide,myristylamide, palmitylamide, stearylamide, and oleylamide; and fats andoils.

Specific examples of the antirusts include metal soaps; polyhydricalcohol partial esters such as sorbitan fatty acid esters; amines;phosphoric acid; and phosphates.

Specific examples of the viscosity index improvers includepolymethacrylate, polyisobutylene, and polystyrene.

The grease composition of the present invention can be prepared, forexample, by mixing and stirring the diurea compounds represented bygeneral formulas (1) to (3) and an organic molybdenum compound, togetherwith other additives if necessary, with a lubricant base oil; andpassing thus obtained mixture through a roll mill or the like. Thegrease composition can also be made by adding the material components ofthe diurea compounds represented by general formulas (1) to (3) to thelubricant base oil beforehand; melting them together; stirring andmixing them so as to prepare the diurea compounds; then mixing andstirring them with the organic molybdenum compound, together with otheradditives if necessary; and passing thus obtained mixture through a rollmill or the like.

The grease composition of the present invention is excellent insuppressing temperature rises and reducing friction, and thus is usefulas a grease for various gears of constant velocity joints, constantvelocity gears, and transmission gears, and various bearings such asball bearings and roller bearings, and is favorably used in constantvelocity joints, bearings for continuously variable transmissions, gearsand bearings for ironmaking equipment, axle bearings for cars/railwayvehicles, etc. in particular.

EXAMPLES

In the following, details of the present invention will be explainedmore specifically with reference to examples and comparative examples.However, the following examples do not restrict the present invention atall.

Examples 1 to 7 and Comparative Examples 1 to 5

Using poly-α-olefin (having a kinetic viscosity of 48 mm²/s at 40° C.)as a lubricant base oil, diphenylmethane-4,4′-diisocyanate (MDI) wasdissolved into the base oil by heating, and monoamines listed in Tables1 and 2, each dissolved in the base oil, were added thereto. Variousadditives listed in the following were added to thus generated gel-likematerials, so as to yield the compositions shown in Tables 1 and 2, andafter stirring, the resulting mixtures were passed through a roll mill,so as to yield the grease compositions of Examples 1 to 7 andComparative Examples 1 to 5. The values of W₁+W₂+W₃ and(W₁+0.5×W₂)/(W₁+W₂+W₃) in thus obtained grease compositions are shown inTables 1 and 2. In Table 2, the cells for W₁+W₂+W₃ in ComparativeExamples 4 and 5 show their respective total thickener amounts insteadof W₁+W₂+W₃.

Additives:

MoDTC (molybdenum dioctyldithiocarbamate)

MoDTP (molybdenum dioctyldithiophosphate)

MoP (molybdenum dibutylphosphate)

Boron type friction modifier (potassium borate type friction modifier)

paraffin oxide (paraffin oxide ester (ester formed between paraffinoxide obtained by oxidizing slack wax and methanol, with a total acidnumber of 33 mgKOH/g and a saponification number of 130 mgKOH/g))

dihydrogen phosphite (diphenyl hydrogen phosphite)

The following temperature rise and friction tests were carried out forthe grease compositions of Examples 1 to 7 and Comparative Examples 1 to5.

Temperature Rise Test

Into a deep groove ball bearing having an inner ring diameter of 55 mm,an outer ring diameter of 90 mm, and a width of 11 mm, 4.0 g of greasewere sealed, and the bearing was rotated under a condition with an innerring rotating speed of 9000 rpm and an axial load of 5000 N. The bearingouter ring temperature at that time was measured, whereby a temperaturerise ΔT was determined. Thus obtained results are shown in Tables 1 and2.

Friction Test

FIGS. 1A and 1B are perspective and top plan views showing a test pieceused in a friction test, respectively. As depicted, a needle holder 2(14 mm×10 mm×2.5 mm) formed at the center part of a lower disc 1 (24 mmin diameter×7.9 mm) was filled with 1 g of each grease composition.Then, three needles (each having a size of 3 mm in diameter×13.8 mm)were accommodated in the needle holder 2, and an upper disc 4 (20 mm indiameter×13 mm) was disposed thereon. These test pieces were set in, anSRV friction tester such that the angle θ [deg] (needle set angle)formed between a line l₁, perpendicular to sliding directions of theupper disc 4, passing the center O of the upper face of the lower disc 1and a line l₂, parallel to the longitudinal direction of the needle 3,passing the center O became 30 deg, and a friction test was carried outunder a condition with a frequency of 40 Hz, an amplitude of 3 mm, aload of 1000 N, and a temperature of 80° C. Table 1 and 2 show therespective friction coefficients obtained 10 minutes thereafter whenusing the individual grease compositions.

Fretting Resistance Test

Using a Fafnir friction oxidation tester, a fretting resistance test wascarried out in conformity to ASTM D4170, so as to measure the amount ofwear. Using a thrust bearing 51204 (manufactured by NSK Ltd.) as abearing, the test was conducted for 2 hours at room temperature. In thistest, the grease compositions made 1 day before were used as samples.Tables 1 and 2 show the results.

Measurement of Consistency

For each of the grease compositions of Examples 1 to 7 and ComparativeExamples 1 to 5, consistency was measured after the lapse of 1 day fromthe making and after the lapse of 3 months from the making. Tables 1 and2 show the results. TABLE 1 Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Example 7 Thickener MDI[mol] 5 10 5 1 10 10 10monoamine p-toluidine 1 9 7 2 9 9 9 [mol] cyclohexylamine 9 11 3 — 11 1111 octadecylamine — — — — — — — W₁ + W₂ + W₃ [mass %] 18 18 18 20 18 1818 (W₁ + 0.5 × W₂) 0.1 0.45 0.7 1.0 0.45 0.45 0.45 {overscore((W₁ + W₂ + W₃))} Base oil PAO [mass %] 80 80 80 78 77 79 77.5 AdditiveMoDTC [mass %] 2 2 2 2 — — 2 MoDTP [mass %] — — — — 5 — — MoP [mass %] —— — — — 3 — boron type friction modifier [mass %] — — — — — — — paraffinoxide [mass %] — — — — — — 2 dihydrogen phosphite [mass %] — — — — — —0.5 Temperature rise ΔT (° C.) 104 94 100 98 97 102 105 Frictioncoefficient 0.090 0.070 0.080 0.085 0.080 0.090 0.080 Frettingresistance (amount of wear [mg]) 2.9 1.6 2.7 2.8 1.8 1.8 0.2 Consistency(60 W) 1 day after making 272 286 295 293 300 291 296 3 months aftermaking 257 282 298 291 305 288 290

TABLE 2 Comparative Comparative Comparative Comparative ComparativeExample 1 Example 2 Example 3 Example 4 Example 5 Thickener MDI[mol] 110 10 1 1 monoamine p-toluidine — 9 9 — — [mol] cyclohexylamine 2 11 111 1 octadecylamine — — — 1 1 W₁ + W₂ + W₃ [mass %] 18 18 18 (18)  (18) (W₁ + 0.5 × W₂) 0 0.45 0.45 0 0 {overscore ((W₁  + W₂  + W₃))} Base oilPAO [mass %] 80 82 80 80   77.5 Additive MoDTC [mass %] 2 — — 2 2 MoDTP[mass %] — — — — — MoP [mass %] — — — — — boron type friction modifier[mass %] — — 2 — — paraffin oxide [mass %] — — — — 2 dihydrogenphosphite [mass %] — — — —   0.5 Temperature rise ΔT (° C.) 138 130135 >150   >150   Friction coefficient 0.120 0.130 0.095    0.130   0.130 Fretting resistance (amount of wear [mg]) 7.0 2.0 2.1   8.5  1.5 Consistency (60 W) 1 day after making 275 281 288 270  276  3months after making 252 283 289 227  214 

As shown in Table 1, it was verified that the grease compositions ofExamples 1 to 7 were excellent in friction characteristics andtemperature rise suppression. Also, the grease compositions of Examples6 and 7 to which a paraffin oxide and a phosphorus compound were addedexhibited a very high level of resistance to fretting.

By contrast, the grease composition of Comparative Example 1 shown inTable 2 was insufficient in terms of friction characteristics, whereastemperature rose greatly when the grease compositions of ComparativeExamples 2 and 3 were used.

INDUSTRIAL APPLICABILITY

As explained in the foregoing, by adding the diurea compoundsrepresented by general formulas (1) to (3) and an organic molybdenumcompound into a lubricant base oil at their respective specificcompounding ratios, the grease composition of the present invention canrealize, at a high level, the suppression of the temperature rise in theearly stage of use and the reduction of the friction.

1. A grease composition containing a lubricant base oil, diureacompounds represented by the following general formulas (1) to (3), andan organic molybdenum compound; wherein respective contents of thediurea compounds represented by the following general formulas (1) to(3) satisfy conditions defined by the following expressions (4) and (5);and wherein the organic molybdenum compound is contained by 0.1 to 20mass % based on the total amount of the grease composition:

where R¹ is a hydrocarbon group containing an aromatic ring, R² is adivalent hydrocarbon group, and R³ is a hydrocarbon group containing analiphatic ring;5≦W ₁ +W ₂ +W ₃≦30   (4)0.1≦(W ₁+0.5×W ₂)/(W ₁ +W ₂ +W ₃)≦1.0   (5) where W₁, W₂, and W₃ arerespective contents (mass %) of the diurea compounds represented bygeneral formulas (1) to (3) based on the total amount of the greasecomposition.